Typically, a turbo machine includes a rotor that extends along and rotates about a central axis with one or more rotor wheels mounted on the rotor and extending radially outward from the central axis. A plurality of buckets or blades are attached to and evenly spaced around the periphery of each rotor wheel. A shroud band may be secured to the outermost radial tips of each bucket on each rotor wheel. A plurality of stator assemblies or diaphragms are also located in the turbine case and are axially spaced from the rotors and extend radially inward from the turbine casing. A plurality of nozzles are attached to and evenly spaced around each stator assembly or diaphragm to direct fluid against and effect rotation of the buckets with the main part of the fluid flowing axially from diaphragm to rotor wheel. Radially outward from the outer radial surface of each shroud band is a surface of either the turbine casing or a surface of an axial extension of the upstream diaphragm.
One of the problems with turbo machines is the leakage of mainstream fluid in the gap between the outer radial surface of the shroud band and the inner radial surface of the extension from each diaphragm. This gap is built-in to allow for thermal expansion of the rotor wheel, buckets, and shroud band and for rotational clearance to allow free rotation of the rotor assembly. Any fluid flow which is lost through these gaps is a loss of power and efficiency for the turbo machine.
Prior attempts to minimize this loss of power and efficiency have had limited success. Typically, most prior designs have focused upon improving non-contact radial sealing and have been less than adequate. Additionally, as noted earlier, there are limitations on how close the gap and thus how close the non-contact radial seals can be constructed because of thermal expansion and clearance requirements. Minor changes in the radial direction often can compromise the integrity of these prior non-contact radial seals. As a result, an unsatisfactory amount of fluid flow continues to be lost.
Another problem with turbo machines is in designing the machines to withstand the dimensional changes which occur due to thermal expansion without allowing mainstream flow to leak from the tips of the buckets. The shroud bands, which seal the top of the buckets, may not expand with heat at the same rate as the buckets. As a result, the shroud bands, buckets, and/or rotor wheels may be damaged when they expand or contract at different rates.
Prior systems have tried to compensate for the expansion by putting circumferentially arrayed gaps into the shroud band to allow the shroud band to expand freely as the machines heat up. Although these gaps compensate and permit expansion of the shroud band and buckets, the gaps are another source of mainstream flow leakage which is a loss of power and efficiency for the turbo machine.
Accordingly, there is a need for a turbo machine which can minimize loss of axial fluid flow and can tolerate, and to some extent, compensate for thermal expansion requirements.